The present invention relates to an arrangement for regulating the layer build-up in the production of optically active thin layer in a vacuum on substrates. Continuous measurement is applied of the optical behavior of the deposited layer. The optical behavior is converted into proportional electrical signals and these signals are subjected to differentiation.
In the present application, "optical behavior" means the influence on the amplitude, phase and spectral dependence of the light used for measurement by the associated layer. When deposited, the characteristics of optically active layers change, e.g., transmission, reflection, phase and polarization state of the measurement light. These effects may be used for measurement or test purposes to the extent described below.
From the German disclosure DT-AS No. 1,548,262 it is already known in the art how to determine the transmission or reflection behavior of the deposited layer by means of a light beam or a pencil of light. The result of such measurement is generally used to interrupt the evaporation process after attaining certain layer properties. However, a very exact interruption of the evaporaton process is not possible, since the obtaining and evaluation of the test results and the interruption of the vapor flow is affected by a time difference so that subsequent growth of the layer cannot be prevented with certainty.
From the German disclosure DT-AS No. 1,214,970 it is further known how to keep track of the layer build-up as a function of time and how to interrupt the evaporation process by observing the transmission of reflection behavior of monochromatic light continuously, and counting the maximums and minimums. The number of maximums or minimums, depending on the wavelength of the light used, gives a clue as to the thickness of the layer. The above publications suggest that, to determine exactly the maximums and minimums, the maximums and minimums of a certain section of the curve be determined by means of a differentiation circuit already known in the art. However, the known conventional arrangement can be used only for those layers having a quarter-wavelength of the measurement light used, or an integral multiple thereof. It is a measurement and regulating procedure for so-called .xi.4 layers.
The concept "optically active layers" within the context of the present applications includes all layers which change the optical properties of the substrate. They may, for example, be reflection reducing layers, filter layers on lenses and other glasses which reflect or transmit part of the electromagnetic radiation in the visible and/or invisible part of the spectrum. The wavelength range of interest in the present invention extends from ultraviolet to the far infrared. The optical activity concerns above all low-loss amplitude changes of the reflected or transmitted radiation. It also includes layers which change the phase or polarization state of the light used for measurement.
Optically active layers may have both a homogeneous composition or, in the individual layer, non-homogeneous composition. They may also consist of a combination of many layers of low and high refraction, as are encountered, for example, with the so-called interference filters. These interference filters have the remarkable property that they extensively compensate errors in the thickness of individual layers as long as the individual layers have the optical thickness of quarter-wavelengths of the light used for the measurement or multiples thereof. However, this requires that the subsequent layer grows together with the preceding until the desired properties are attained. Hence, not the properties of the individual layer, but the effect of the totality of the layers is decisive as to the effect achieved. From this follows that especially multiple layers of the type described, can be produced only by the application of optical measurement methods, -- not mechanical methods -- with the desired close tolerances.
In view of the overall properties of optically active thin layers, recently increasingly tight tolerances have been required. This assumes that the production process for these layers is to a high degree reproducible, in order to obtain with equal success layers of constant quality. This applies not only to the numerous surface layers of complex optical systems, but also to spectacle lenses, especially sunglass lenses. It is self-understood that, for example, in the case of breakage of a spectacle lens, a lens with different optical properties cannot be considered. Color differences in particular are not permissible.
For certain layer combinations, as used, say, for wideband reflection elimination layers, not only layer thicknesses which are integral multiples of quarter-wavelengths are of interest, but also those whose thickness is a multiple of .lambda. /8.
It is, therefore, an object of the present invention to provide an arrangement of the initially described type in which .lambda./8 layers of high reproducibility of all properties can be obtained with simple operation of the evaporation device.
Another object of the present invention is to provide an arrangement of the foregoing character which is simple in design and construction, and which does not require skilled scientific personnel to operate.
A still further object of the present invention is to provide an arrangement, as described, which has a long operating life and may be maintained in service economically.